This only works in cases where the IPv4 address is really encoded in the IPv6 one at a known location. IPv6 addresses might not encode ANY IPv4, any even if they do, they don't always have to follow a specific scheme. For example, on my old server, the last 5 numbers, decoded to decimal, where in the form of:

10 0 0 138 2

Where the last digit was the number of the virtual machine or service i wanted to have their own IPv6. Basically, i shifted the whole IPv4 one byte to the left, so i could have sub-numbering of stuff running on that IPv4 address.